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Seismic Interpretations

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CHAPTER 2 SEISMIC INTERPRETATIONS

2.1 INTRODUCTION

The first Chapter of this training material was on the topic of geology. In it we stressed the importance of understanding geological principles because our end product - the seismic section - is

a representation in time of the geology.

This course would not be complete if we did not have a look at the interpretation of seismic data.

No matter what our particular job is in the industry, we should all have an understanding of the problems forced by the interpreter, whose job it is ultimately to identify a feature that is a possible

drilling creation.

In this chapter, then, we will look first at the tools used by the interpreter, and then do a couple of

interpretation exercises on data from two completely different tectonic regimes. Hopefully these exercises will bring the course to a satisfactory conclusion, and help you to understand how the

seismic tool images the geology.

2.2 WELL LOGGING TOOLS FOR THE GEOPHYSICIST

Geologists make use of a number of logging tools - tools that are lowered into a well in order to

make in-situ measurements of various parameters within the formations. Such parameters as

interval velocities, density, electrical resistance and gamma ray activity all provide useful information about the rocks at depth. As the source - receiver distance in a logging tool tends to be

quite short, the information gained has a high frequency content and is hence high resolution.

2.2.1The Sonic And Density Logs

The most important tools for the geophysicist are those used to obtain interval velocities and

formation densities (why?).

For obtaining interval velocities we use the sonic tool. The sonic tool consists of a source of energy and a receiver, separated by some distance (typically a few metres), mounted in a tool that is

clamped to the side wall of the well bore so that there is good contact with the formation. The

standard tool in use today is the Borehole Compensated tool - this has two transmitters and four receivers (see Figure 2.1) in such a configuration that borehole irregularities, and tilt of the tool are compensated for. Measurements are made as the tool is drawn up from the bottom of the well. The data is presented as a continuous trace (Figure 2.2), calibrated in depth and in transit time - that is, in units of microseconds per foot, or per metre. From these measurements can be derived interval velocities within the formations.


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The other parameter important to the geophysicist is formation density. Densities are measured

using a gamma-ray source and a detector shielded so that it records only back-scattered gamma radiation from the formation (Figure 2.3). (The figure is that of a compensated detector; it employs a second, short range detector, which responds more to the mudcake and small amplitude bore-

hole irregularities; these reading are used to correct the readings from the main detector.) The intensity of the back-scattered radiation depends upon the electron density of the formation, which

is roughly proportional to the bulk density. The form of presentation of the density log is shown in Figure 2.4. So we have two parameters measured within the well. What do we do with them?

Figure 2.1 THE SONIC TOOL 1


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Figure 2.2 THE SONIC LOG


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Figure 2.3 THE DENSITY TOOL


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Figure 2.4 THE DENSITY LOG


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2.2.2 The Synthetic Seismogram

The reflection coefficient series (which we also refer to as the earth impulse response) derived at a

well location represents an important calibration point for our surface seismic measurements. It is only at a well that we can make real measurements at depth; measurements that, if we are lucky,

we can calibrate against actual rocks pulled from the well as core samples. Even without core samples, however, well logging is vital to our understanding of the formations that we "see" on a

seismic section.

Having obtained our reflection coefficient series, what can we do with it? The clue lies in the

operation of convolution - recall that the seismic section is a result of convolving the seismic wavelet with the earth impulse response. In collecting our surface seismic data, we know little

about the source wavelet, and nothing about the impulse response. All our measurements are made after the fact, so to speak. However at the well, we have actually measured the real impulse

response (within the limitations of the logging technique; nothing is ever perfect). Why not design a

reasonable wavelet (i.e. design a filter) and convolve it with the measured impulse response?

The result is the synthetic seismogram - basically a model of what we would expect a seismic trace to look like at this location given a similar source wavelet (see Figure 2.5). The choice of which wavelet to use for generating a synthetic seismogram is a judgment call for the

interpreter to make based on a knowledge of the area. It is often useful to test different wavelets,

and to output a number of synthetics using different frequency wavelets, then testing to see which model fits the seismic section the closest. For southern Alberta, the Orsmby wavelet is often the

best to use.

2.2.3 The Checkshot Survey

In order to obtain better information about seismic velocities at a well location, a check-shot survey

will be carried out. The idea is relatively simple: lower a geophone into the well, then trigger a seismic source close to the well location and measure the first arrival time; this will give a direct

measurement of the average seismic velocity to that geophone level at the well, and this

information can be used to convert seismic time to seismic depth.

An actual check-shot survey will take measurements at a number of depths within the well. The geophone spacing within the well will be in the order of hundreds of metres, and only the first

arrival information is used (see Figure 2.6). Included in this figure is the most important information that the check shot survey gives the interpreter - the time-depth curve at the well.


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Figure 2.6 CHECK SHOT SURVEY - SCHEMATIC


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2.2.4The Vertical Seismic Profile (VSP)

A natural and most useful extension of the check-shot survey is the VSP. The recording technique is

essentially the same except that many more levels are recorded, and the recording time is extended to 3 or 4 seconds. A VSP will typically be recorded every 25 metres within the well. This means that

a VSP is much more expensive to record (more rig time is required), and hence they are not done

often. Depending on the TD of the well, and the number of levels recorded, a VSP can run from $50,000 to $150,000. The big advantage of the VSP is the amount of information it gives the

geophysicist. The principle of VSP acquisition, and a VSP plot are given in Figure 2.7 and Figure 2.8 respectively.

Figure 2.7 VSP - SCHEMATIC


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Figure 2.8 VSP - PLOT


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2.3 POLARITY AND ITS IMPORTANCE

The problem of polarity is as old as the seismic method itself. The geophysicist needs to know the

polarity of the section in order to help determine the stratigraphy of the section. Polarity has been defined by the SEG (see definitions) and is important for the following reason: we define a

reflection coefficient as positive at an interface where a compressive wave is reflected as a compressive wave. This implies an increase in seismic velocity from the first medium to the second,

and the geophysicist (or geologist) will then know the kind of stratigraphic change taking place.

Hence it is vital to know what a peak or a trough is truly representing on the seismic section.

The main tool the interpreter has to determine polarity is the synthetic seismogram, in which the reflection coefficient series is convolved with a known wavelet, and then displayed at both polarities

for that wavelet. The seismic section that truly (or most closely) matches the synthetic derived with the wavelet of positive polarity will be designated as the "log normal" section. Polarity becomes

especially critical in stratigraphic plays. Basing one's interpretation on the wrong polarity can mean

the difference between drilling success and failure (why should this be?).

2.4 THE STACK VS. THE MIGRATED SECTION

The final interpretation done by a geophysicist will almost always be done with the migrated

section; it is vital to have any structural elements and faults placed in their correct spatial location relative to each other and relative to known geographical coordinates. The final objective of the

interpretation is a drilling location; basing a location on non-migrated data would be foolish at best, disastrous at worst.

However the stack section is very useful in two situations that come to mind immediately; in highly structured and faulted areas and in reef plays.

In both types of plays, it is vital to pinpoint exactly the edge of a fault, or the edge of a reef. These

are often marked by the apex of diffraction patterns, which of course are collapsed in a migrated

section (that is if the processor has done the job correctly). Thus, although the interpreter will want to base the mapping of the play on migrated data, he or she will also want to interpret the stacked

section and mark diffractions as an aid to pin-pointing the edges of structures.

2.5 TIME TYING SEISMIC DATA

A familiar problem facing any interpreter is that of seismic lines that do not tie properly at their

intersections. The problem is most acute when trying to tie data of different vintages or from different surveys that have used different sources or different receivers or different recording

systems, or different processors.

Source Problems

When tying dynamite data to Vibroseis data you are probably facing a phase difference in the two

data sets: dynamite is minimum phase and Vibroseis closer to zero phase. A phase difference between two data sets means a mistie. The problem lies in the fact that a reflection peak on one

line will be phase shifted to a slightly different time from the peak of the same reflector on the

other line.


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Receiver Problems

When dealing with mistying data sets, look at the header on the sections to see if the geophones

were of different type. If so, they may have different frequency responses and may contribute again to a phase difference between the sections.

Recording Problems

Different recording systems will introduce different phase problems into the data.

Datum Elevations And Replacement Velocities

You will often find that different vintages of data have used different datum elevations and/or

different replacement velocities so immediately there will be a mistie.

Processing

If you examine the headers of the different vintages of data, you will undoubtedly find that different

processing streams have been used; different types of decons, and different filtering, have been

applied. This will again affect the phase of the data as well as the frequency content and hence the data probably won't tie.

So - given all these factors that combine to make two data sets mistie, what do you do about it?

One solution (the one that keeps processing houses happy) is to take the complete data set - all

vintages - and reprocess the complete set with the same processing parameters. It may be necessary to apply some kind of phase matching process as well. Even reprocessing all the data to

the same elevation, using the same decon and filters and phase matching may still leave you with a residual mistie problem.

Here then we must consider that the end product wanted by the interpreter is a drilling location. A location may be picked on the basis of structural mapping, with the faults in their correct map

position. If the misties are minor enough - or if, say, it is only one or two lines that mistie, then it may be possible to apply a bulk shift to bring the mistying data into line. However a structure map

based on data with significant bulk shifts will always be suspect. The only real solution is to map isochrons; even if two lines misty in time, the isochrons between reflectors should be the same. We

deal with isochron mapping later on in this Chapter.

2.6 UNCONFORMITIES AND THEIR SEISMIC EXPRESSION

We touched on the unconformity in Chapter 1 on geology. Unconformities are important markers

for the geologist and the geophysicist. There are some major world-wide unconformities that are

recognizable in seismic data from different parts of the globe; one is the mid-Cretaceous unconformity. In Western Canada there is a major unconformity at the base Mezozoic. These

unconformities are usually good seismic markers because they mark a boundary between older rocks and much younger ones. A period of non-deposition or erosion before a renewal of deposition

means the boundary is between different rock types with markedly different velocities and densities,


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producing a large reflection coefficient. This can give rise to a strong seismic reflector that in some

places can be followed for many kilometres.

The angular unconformity gives a distinctive seismic expression; underlying dipping reflectors can

be seen to truncate against the unconformity.

2.7 FAULTS AND THEIR SEISMIC EXPRESSION

We have previously discussed the different kinds of faulting that we observe in the geological

record. We find these faults at depth, and they have a seismic signature, or expression, that is sometimes obvious and sometimes not. A fault plane can cut across reflections on a seismic section,

and the displacement can be obvious. Sometimes if the fault cuts the seismic line at an oblique angle the displacement may not be so obvious; and a low angle fault might appear as a reflection

event not readily identifiable as a fault.

2.8 REEFS AND THEIR SEISMIC EXPRESSION

The seismic detection of reefs can be difficult. There is often a relatively small velocity contrast

between the reef and the surrounding rock, so there is only a small reflection coefficient. There are

some clues on a seismic section, however, that tell us a reef is present. There may be isochronal thickening: that is, in the vicinity of the reef, there will be an overall thickening in the time thickness

of the strata; there may be as well isochronal thinning in the strata immediately over the reef as a result of differential compaction. Another clue as to the presence of a reef can be diffractions

originating at the reef edge. Because the seismic velocity within the reef is somewhat higher than in the surrounding rock (especially if the reef is embedded in shale) then we may observe the

phenomenon known as velocity pull-up; horizons below the reef will appear earlier in time than they

would if the reef was not there. This effect has lead to interpreters drilling apparent structures that are not real.

Another clue to the presence of the reef is that a reef may exhibit a total lack of internal

structuring, so that reflectors disappear below the crest of the reef structure. There can also be a

"focusing" effect below the reef.

2.9 TIME STRUCTURE MAPPING

Once the seismic sections are all interpreted, the geophysicist will want to view the data in map

form. The basic map is the time structure map - that is a map of time values for the horizon with time zero being the seismic reference datum. On such a map, the larger the value of time the

deeper the horizon, so structural highs, which may represent potential drilling locations, have lower relative values. A number of maps might be prepared for successively deeper horizons. The map

will be prepared with the seismic time values being posted to the map and then contoured. Such

posting and contouring may be done by computer or by hand. Computer contoured maps will often be hand revised as the automatic programs do not always handle the data correctly.


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2.10 THE IMPORTANCE OF THE ISOCHRON

In our search for hydrocarbon traps, we need to find traps that were in place before the migration

of hydrocarbons into the trap. If we see a structure on a seismic section we must ask if the structure is real for that time of deposition or not, and one tool to help us in this is the isochron.

An isochron is a surface of equal time difference; for example, a map of a particular horizon is an

isochron from time zero to the time of the horizon. It is more instructive, however, to make an

isochron map of the time from some known reference horizon down to our horizon of interest. If the isochron thickness remains the same over the apparent structure, this tells us that the structure

is not real - or at least it was not in place at the time of deposition of the reference horizon.

In other words, the isochron tells us the geomorphology of the surface of interest at the time that the reference horizon was being deposited; we make the basic assumption that the reference

horizon was deposited horizontally, and taking the isochron is in effect flattening the reference

horizon. Figure 2.9 attempts to illustrate how isochronal thinning over a structure is a clue to the fact that the structure is real.


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Figure 2.9 HORIZON FLATTENING - DIAGRAM


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2.12 DEFINITIONS

Polarity - The SEG standard for seismic data states that the onset of a compressional pulse

is represented by a negative number. Positive polarity for a seismic wavelet relates to an

increase in acoustic impedance, or a positive reflection coefficient. For a zero-phase wavelet, a positive reflection coefficient is represented by a central peak,

normally plotted black.

Unconformity - A surface of erosion or non-deposition that separates younger strata from older

rock. Often forms a good seismic reflector and may be angular or not

Anisotropy - Velocity parallel to the bedding plane different from the velocity perpendicular to the bedding plane

Anomaly - Deviation from uniformity in physical properties

Anticline - A fold in strata in which the rocks dip in opposite directions away from the crest. The layers are convex upward. Opposite of a syncline

Antithetic fault - A secondary fault having a throw in the opposite direction to that of the

main (synthetic) fault with which it is associated

Bright spot - A local increase in amplitude on a seismic section

Compaction - Loss of porosity with pressure, usually in a non-elastic way. Differential

compaction results from uneven settling of sediments due to differences in the irreversible

volume change that takes place as the result of pressure from overlying sediments.

Conformable - Two sedimentary beds, parallel to each other, separated by a surface of original deposition, with no disturbance or erosion having taken place during their deposition

Correlation - The identification of the phase of a reflection on a seismic trace being the same

as that on another, thus identifiying the two reflections as being one and the same. A jump

correlation is done between two seismic lines that do not intersect

Density - Mass per unit volume

Diffraction - The bending of wave energy around obstacles; the result is that wave energy

penetrates into areas forbidden by geometric optics

Flower structure - Geological structure resulting from strike-slip movement associated with convergence. Characterized by high angle reverse faults with antithetic faulting; the fault

pattern appears to "branch" vertically

Horizon - The surface separating two rock layers

Interval transit time - Travel time of a wave over a unit distance. In well logging, measured in

microseconds per metre (or per foot)

Isochron - Line connecting equal reflection times on a map


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Isopach - Line connecting points of equal rock thickness

Mis-tie - The difference of values at identical points on intersecting lines

Monocline - An area in which dip is everywhere the same direction, although not necessary

the same amplitude

Multiple - Seismic energy that has been reflected more than once

Paleo - Referring to some past time. Thus a paleosurface is a surface that existed at some

time in the past. A paleosection is one in which the bedding is represented as it may have looked at some time in the geological past

Pull-up - An apparent uplift produced by a shallower high-velocity layer

Rarefaction - Temporary separation of molecules as result of the passage of a P-wave

Transducer - A device which converts one form of energy to another Well log (Borehole log) - A record of one or more physical parameters as a function of

depth in a well